Hydraulic fluids used in supersonic aircraft, missiles, and space crafts and materials used as brake fluids must meet very demanding performance criteria. The extreme variation in temperatures encountered between the high temperature of space travel re-entry and the low temperature of space severely limit the number and type of fluid materials which can be employed. Such temperature changes usually produce dramatic viscosity changes in fluids used in the craft which subsequently require compensation by mechanical parts. Frequently, such compensation is impractical or impossible and a substitute fluid must be found.
One class of fluid materials previously investigated as lubricants and hydraulic fluids for missiles and space craft includes silahydrocarbons, SiR.sup.1 R.sup.2 R.sup.3 R.sup.4, where R is an alkyl group of one to twenty carbon atoms. Various materials within this class were found to have desirable properties of thermal stability up to 700 degrees F.; stable viscosity at temperatures ranging from -65 degrees to 400 degrees F.; excellent oxidative stability; low freezing points; and excellent lubricity.
However, silahydrocarbons are difficult and expensive to prepare. An inexpensive chemical synthesis of economic hydraulic, lubricating, and brake fluids, with all or most of the properties of silahydrocarbons, is therefore needed.
Alkylmethylpolysiloxanes have been investigated for such applications. The benefits of silicones for uses as hydraulics, lubricants, and brake fluids were recognized early in the development of these unique materials. McGregor et al., U.S. Pat. No. 2,398,187, issued Apr. 9, 1946, disclosed the broad generic use of linear dialkyl siloxane polymer and copolymer fluids for such applications on the basis of their slight change in viscosity with temperature, low pour point and high flash point. In addition, McGregor et al. recognized other beneficial properties, including: low volatility, low hygroscopicity, little or no corrosive or decomposing effect upon metal and rubber hydraulic device components, and little or no gasification or solidification tendencies under the higher or lower temperature conditions encountered in the various types of hydraulic applications.
More recently, commercial attention has focused on polydimethylsiloxanes (PDMS) as viable alternatives to the glycol-based polyethers traditionally employed in many hydraulic systems. This is not surprising, since PDMS represents the most ubiquitous and inexpensive silicone of manufacture. PDMS will not absorb or dissolve a significant amount of water and is amenable to lubricity additives. However, PDMS fluids have a disadvantage of a relatively high (about -50 degrees C.) solidification temperature which precludes their use in certain applications. Such disadvantage was engineered out of these fluids by copolymerizing mono- and trifunctional units with the difunctional siloxanes to attain a non-regular structure, i.e., a branched siloxane, with a greatly reduced tendency to crystallize. An example of a non-linear siloxane was disclosed by Holbrook et al. in U.S. Pat. No. 4,137,189, issued Jan. 30, 1979. In this case, the object was to provide an hydraulic fluid which could be utilized in all fluid transmission systems of an automobile. This application demanded greater lubricity on metals than ordinarily required in brake fluid applications. Thus Holbrook et al. disclosed a composition consisting essentially of a branched siloxane fluid, a chlorendate diester and a lubricant additive selected from dithiocarbamates and phosphorodithioates of antimony and lead.
A variation on the teachings of Holbrook et al. was disclosed by Keil in U.S. Pat. No. 4,443,351, issued Apr. 17, 1984, wherein it was suggested that the less expensive, linear polydimethylsiloxanes, or copolymers of PDMS with alkylmethyl siloxanes, be blended with a chlorendate diester, a lubricant additive (as above) and a block copolymer of polydimethylsiloxane-polybutadiene.
Brenner, in U.S. Pat. No. 3,671,433, issued June 20, 1972, disclosed the enhancement of alkylmethyl polysiloxane fluid lubricity with the addition of small amounts of dodecenyl succinic acid. It was shown that when the alkyl groups contained from 6 to 18 carbon atoms superior lubrication properties resulted. Brenner, however, does not teach the use of cyclic alkylmethylsiloxanes as in the instant invention.
Many patents address the preparation and use of cyclic polysiloxanes. However, to the knowledge of the applicants, no one has taught the preparation of the mixed methylalkyl tri- and tetracyclosiloxanes of the instant invention wherein each silicon atom has bonded to it one methyl group and one alkyl group and wherein two or more different alkyl groups larger than methyl are also bonded to silicon atoms in the cyclosiloxane.